Some electromagnetic flow meters of the related art are configured to extract an electromotive force generated in a fluid flowing in a measurement tube by using an electropotential detection electrode. The electropotential detection electrode, typically made of a stainless steel, is made of various materials depending on corrosiveness of detection targets. Examples of electrode materials having high corrosive resistance include precious metal materials such as platinum as disclosed, for example, in PTL 1 in many cases. The precious metal materials have such a disadvantage as having difficulty to achieve desired shapes depending on the shape due to their low strength. In order to solve the disadvantage as described above, forming an electrode by using a non-precious metal material as a base metal and covering the electrode with a precious metal material is conceivable as described in PTL 1.
However, when an abrasive substance is included in the fluid flowing in the measurement tube, the precious metal material covering the electrode may be peeled off. In addition, the precious metal material covering the electrode may be peeled off due to an impact applied to the measurement tube, corrosion of the measurement tube, or a defect at the time of manufacture.
When the precious metal material is peeled off, an electrochemical noise is generated due to an electropotential difference between the base metal and the precious metal material. This noise corresponds to an output noise from the electromagnetic flow meter.
The problem of generation of the noise due to peeling off of the metal material that covers the electrode may be solved by making an electrode body with an insulating material, covering a surface of the insulating material with a precious metal material, and using the precious metal material as a conduction path as proposed by an applicant of the present application in PTL 2. Described in PTL 2 is an electropotential detection electrode 3 having such structure that a base material 1 made of a ceramic, which is an insulating material, is covered with a conductor 2 made of a metal having corrosive resistance as illustrated in FIG. 8. The electropotential detection electrode 3 includes a first small diameter portion 3a to be inserted into a hole 5 formed in a measurement tube 4 for inserting an electrode, a large diameter portion 3b located outside the measurement tube 4, and a second small diameter portion 3c projecting from the large diameter portion 3b in a direction opposite from the first small diameter portion 3a. A lead wire 6 is connected to the second small diameter portion 3c. 
The large diameter portion 3b may be provided with an annular groove 7, as illustrated in FIG. 9. In FIG. 9, the same or similar members as or to those described in conjunction with FIG. 8 are denoted by the same reference signs and detailed description will be omitted. The annular groove 7 is configured to accommodate a gasket 8, and is formed into an annular shape and located on the same axial line as the first small diameter portion 3a. The gasket 8 is configured to seal between a fluid path 4a in the measurement tube 4 and the large diameter portion 3b of the electropotential detection electrode 3.
In this manner, the conductor 2 of the electropotential detection electrode 3 provided with the annular groove 7 is preferably formed by using a metal paste (not illustrated) so that an inner peripheral surface 7a, a bottom surface 7b, and an outer peripheral surface 7c of the annular groove 7 are uniformly covered. The metal paste contains metal powder, and is formed into the conductor 2 by being sintered together with the base material 1 in a state of being applied to the base material 1 and being subjected to metal paste sintering.